Calculation
Rsh = Umovement/(Itotal−Imovement)
Movement and shunt have the same voltage drop; the shunt carries total current minus movement current.
Busch 2006, Abschnitt 10.3 und Aufgabe 10-10
Ammeter range-extension shunt
Movement and shunt have the same voltage drop; the shunt carries total current minus movement current.
MINTSI
02
Result
Select a target and calculate.
Understand the inputs
- Required shunt resistance Rsh — Low-value parallel resistor including the effective connection resistance.
- Original movement full-scale current Imovement — Current through the unchanged movement at full deflection, not the desired new total current.
- Full-scale burden voltage Umovement — Voltage directly across the movement at original full scale; the same voltage will appear across the shunt.
- New full-scale current Itotal — Total current that should just produce full deflection; it must exceed Imovement.
Example
A 10 A movement with 120 mV burden extended to 100 A gives Ish=90 A and Rsh≈1.333 mΩ.
Assumptions and limits
Ideal current division at equal terminal voltage; temperature coefficient, contact resistance, pulse loading and calibration are excluded.
Understand Ammeter range-extension shunt
This calculator sizes the low-resistance parallel path that lets an existing ammeter movement measure a greater total current without increasing its allowed movement current.
What does this quantity describe?
At full deflection, Imovement flows through the instrument and Itotal−Imovement through the shunt. Since they are parallel, both have the same burden voltage.
Formula and variables
Rsh = Umovement/(Itotal−Imovement)
Ish = Itotal−ImovementRsh = Umovement/IshPsh = Umovement·Ish
| Symbol / input | Meaning |
|---|---|
| Required shunt resistance Rsh | Low-value parallel resistor including the effective connection resistance. |
| Original movement full-scale current Imovement | Current through the unchanged movement at full deflection, not the desired new total current. |
| Full-scale burden voltage Umovement | Voltage directly across the movement at original full scale; the same voltage will appear across the shunt. |
| New full-scale current Itotal | Total current that should just produce full deflection; it must exceed Imovement. |
Choose the inputs correctly
Imovement and Umovement are a matching pair from the data sheet, scale or a measurement of the existing instrument. Itotal is the desired new full-scale value and must be larger.
How to use the calculator
First establish movement voltage drop at original full scale, then choose the new range. After resistance, also check real-shunt power, connection method and temperature drift.
Worked example
With Imovement=10 A, Umovement=120 mV and Itotal=100 A, 90 A flows through the shunt. Rsh=0.12 V/90 A=1.333 mΩ and shunt loss is 10.8 W.
Understand the result and units
Such a small resistance effectively includes lead and contact resistance. Fractions of a milliohm can measurably alter current division.
Burden voltage may be entered in mV and the result is conveniently shown in mΩ. Current values must refer to the same full-deflection condition.
Typical applications
Analogue ammeters, external test-bench shunts and teaching current division.
Assumptions, limits and common mistakes
Steady DC and ideal parallel terminals. AC inductance, self-heating, Kelvin connection and pulse energy are not sized.
Common mistake: Only Itotal−Imovement, not all of Itotal, flows through the shunt. Never select the resistor without power and temperature checks.
Frequently asked questions
What is “Ammeter range-extension shunt” used for?
Analogue ammeters, external test-bench shunts and teaching current division.
Where do the input values come from?
Imovement and Umovement are a matching pair from the data sheet, scale or a measurement of the existing instrument. Itotal is the desired new full-scale value and must be larger.
What does the result not cover?
Steady DC and ideal parallel terminals. AC inductance, self-heating, Kelvin connection and pulse energy are not sized.